Time frame depends completely on you.
If, you are able to put in a full days work on your bus it can be done very quickly pending what you want it to be in the end.
If, you are like most of us and work. You will focus on weekends and the rare mid-week evening to hammer away on projects.
My skoolie started out roaring, I bought it in march and finished one major project per month for the next spring, summer and fall before packing up for the northern minnesota winter.
I reskinned my entire bus, fabricated an entirely new rear end, I lifted the roof, I lowered the skirt, I had to replace sections of bare metal flooring, removed windows, skinned over the windows after the lift, the list goes on and on. Plus, I gave it a full coat of protective primer between March 2016 and November 2016.
This summer has been a different story, I bought a surplus Humvee and restored it to a customized state which basically robbed six months of work and nice weather from my skoolie. Since July, I have installed conduit, roughed in windows, lined the ribs with cedar, installed the sub floor and rough floor and enclosed the rear service entry. It has been a busy month.
I will have the wiring, plumbing, electrical, windows, temp interior walls and major builds done and ready for next spring when I can spray foam.
At my prior and previous rate, I will have my bus done in about nine years. Just kidding, I would say by Spring 2018. It is basically a frame up restoration so, I am not a quick fix, lets rock it out so we can rocket out of here kinda guy.
Based on your $4,000 budget I would say you could have it done in a couple of months. It would be pretty rough, pretty wide open but, you and your family could adjust and live out of it. It might not be comfortable or have onboard dry cleaning but, damn it you can make it work.
D.L. Jones III